This invention relates in general to inkjet printheads and, more specifically, to control in the directionality of ink drops ejected from a printhead in order to improve image quality. More particularly, the invention relates to a method of modifying a nozzle cavity space so as to compensate for the effects of defects in an inkjet printhead by altering the direction of ink drops ejected from a nozzle.
Without limiting the scope of the invention, its background is described in connection with inkjet printers, as an example.
Modern color printing relies heavily on inkjet printing techniques. The term xe2x80x9cinkjetxe2x80x9d as utilized herein is intended to include all drop-on-demand or continuous inkjet printer systems including, but not limited to, thermal inkjet, piezoelectric, and continuous, which are well known in the printing industry. Essentially, an inkjet printer produces images on a receiver medium, such as paper, by ejecting ink droplets onto the receiver medium in an image-wise fashion. The advantages of non-impact, low-noise, low-energy use, and low cost operations, in addition to the capability of the printer to print on plain paper, are largely responsible for the wide acceptance of inkjet printers in the marketplace.
The printhead is the device that is most commonly used to direct the ink droplets onto the receiver medium. A printhead typically includes an ink reservoir and channels, which carry the ink from the reservoir to one or more nozzles. Typically, sophisticated printhead systems utilize multiple nozzles for applications such as high-speed continuous inkjet printer systems, as an example. Continuous inkjet printhead device types include electrostatically controlled printheads and thermally steered printheads. Both printhead types are named according to the means used to steer ink droplets ejected from nozzle openings.
It is well known in the art of inkjet printing that image quality suffers from a failure to accurately control the direction from which ink drops exit the printhead. Variations in the direction of ink drops ejected from a given nozzle from a desired direction of ejection (usually perpendicular to the printhead surface) can occur due to changes in the nozzle during operation, as a result of manufacturing defects present before operation, or both. In most instances, repairs are too difficult and costly, resulting in scrapped parts and decreased manufacturing yields. Accordingly, a cost effective way of increasing printhead lifetimes and printhead production yields would be advantageous.
For any given nozzle, the direction of the exiting ink drop stream is controlled by the physical characteristics of the nozzle. Where misdirection occurs, the ink drops can produce printing artifacts such as random placement errors between subsequent drops from a single nozzle or placement errors of drops from one nozzle with respect to those from another nozzle. Variations in the direction of ink drops ejected from a given nozzle may occur over a variety of time scales. For example, in Bubble Jet printheads, made by Canon Company, rapid variations may occur when bubbles nucleate randomly on the surfaces of heaters, causing random variations in the velocity and direction of ejected ink drops from each nozzle. Variations in the direction of ejected ink drops may also be caused by sources external to the inkjet printhead such as, for example, vibrations of the inkjet printer. It is difficult or impossible to correct such random variations in the direction of ejected ink drops, which typically change rapidly with time.
In other cases, factors causing deviation of the direction of ejected ink drops from a desired direction can occur slowly over a long period of time. Such slowly changing variations may arise, for example, from gradual changes in the material properties of the nozzle, such as changes in the stress of the materials comprising the nozzle or surrounding the nozzle openings, from changes in the resistance of heater materials during operation, or from wear of nozzle materials during operation.
In still other cases, factors causing deviation of the direction of ejected ink drops from a desired direction can be essentially permanent. Deviations caused by manufacturing defects in nozzles, for example defects that alter or vary the shape of the nozzle openings, are essentially permanent. Permanent deviations may also arise after a period of time of operation of a nozzle. For example, a piece of material may become permanently chipped away from a portion of a nozzle after a period of time of operation, or a piece of material may lodge permanently within a nozzle during operation.
Thus, it is desirable to compensate for slowly changing variations in the directionality of ejected ink drops. For slowly changing variations, compensation may be needed from time to time during operation. It is also desirable to compensate for permanent changes in the directionality of ejected ink drops in order to improve image quality and increase manufacturing yield. Compensation cannot be applied before operation of the nozzles, since it is generally not possible to predict the direction and magnitude of deviations in the direction of ejected drops for a particular nozzle, which occur after operation. Compensation applied after or during operation of nozzles is herein referred to as active compensation.
Substantial effort has been directed toward active compensation for slowly changing variations in the direction of drop ejection for drop-on-demand printers, as discussed and illustrated, for example, in U.S. Pat. No. 4,238,804, assigned to Xerox Corporation, and U.S. Pat. No. 3,877,036, assigned to IBM, which teach measuring the position of ejected ink drops and compensating for variations from the ideal direction by electrostatic means. While such electrostatic deflection can be used to direct ink in a desired direction, as is well known in the art, electrostatic deflection in these cases adds mechanical complexity. Also, correction techniques of this type are largely ineffective in cases where large variations in the direction of ejected ink drops occur.
U.S. Pat. No. 5,592,202, assigned to Laser Master Corporation, teaches an electronic means to correct inaccuracies in ink drop placement by advancing or retarding the time of a drop-on-demand actuation pulse. However, this method does not correct variations in both of the directions of ink drop ejection in a plane perpendicular to the direction of drop ejection, as it is more suited to adjusting ink drop placement only in the scan direction of the printhead. Moreover, not all printhead circuits can be easily adapted to control the firing times of individual ink drops, since the firing pulses may be derived from a common clock.
U.S. Pat. No. 5,250,962, assigned to Xerox Corporation, teaches the application of a moveable vacuum priming station that can access groups of nozzles to remove entrained air in one or more nozzles. Although entrained air is known in the art to cause variations in the direction of ink drop ejection, it is only one of many mechanisms causing variations. Also, entrained air principally refers to failure of the ink to fill the printhead, not to a change in the head itself. Removal of trapped air serves to restore the nozzle to its original condition, but does not alter the physical characteristics of the nozzle.
Other prior art techniques for achieving compensation include the selection of one nozzle among a plurality of redundant nozzles for printing a particular imaging pixel, the preferred nozzle having favorable ink drop ejection characteristics. However, redundancy selection techniques of this type are complex in nature and require substantial real estate space on the printhead form factor to implement. Such methods also increase cost and/or reduce productivity.
In the case of continuous inkjet printheads using electrostatic steering of ink drops, as in the current generation of commercialized continuous inkjet printheads, for example those manufactured by Scitex Corporation, compensation for variations in the direction of ejected ink drops from an ideal direction can be accomplished by electrostatic means; and in this case, additional mechanical complexity is not required, since the means of printing itself is based on electrostatic deflection and the required hardware is already in place. Printheads of this type produce electrically charged ink drops, which are deflected using a charged electrode at each nozzle. The electrode voltage is set to one of two discreet values (for example, either 100 volts or 0 volts) each time an ink drop is ejected, causing ink drops to be deflected either in a printing direction (for example, in the case the voltage is 100 volts), or into a gutter (for example, in the case the voltage is 0). To correct for slow or permanent deviations of the direction of ejected drops from a particular nozzle, the voltage corresponding to printing at that nozzle might be set, for example, to 110 volts. The use of electrostatic techniques such as these, however, requires additional voltage control hardware.
In the case of continuous inkjet printheads using thermal steering of ink drops, an electrode apparatus is not already in place, and other means of correction are desired to correct for the effects of slow variations in direction of ink drop ejection, as well as for permanent manufacturing defects.
Accordingly, a need exists for a cost effective method of correcting defects in inkjet printheads to permit compensation in the direction of ink drops ejected from the nozzles. A means of increasing manufacturing yields by permitting active compensation for ink drop ejection misdirection from a nozzle would provide numerous advantages.
The present invention provides a system and method of modifying nozzle cavity space in order to compensate for the misdirection of ink drops ejected through at least one nozzle of the printhead. With the present invention, printheads that would normally be discarded due to defects that cause ink drop misdirection can be repaired rather than discarded.
Accordingly, disclosed in one embodiment is an inkjet printhead with integral compensation for misdirection of ink drops ejected through at least one nozzle of the printhead. The inkjet printhead comprises a substrate forming a wall, which defines a nozzle adapted for facilitating the flow of ink from an ink reservoir to a nozzle cavity having a nozzle opening via an ink stream pathway. The inkjet printhead also comprises a membrane predisposed over the nozzle having a nozzle cavity to create a resistive barrier against ink flow. The membrane includes the nozzle cavity and the nozzle opening through which ink drops are ejected.
The inkjet printhead further comprises at least one reservoir integrated within the membrane and a channel formed within the membrane underlying an upper layer and extending from the reservoir to a nozzle cavity. The reservoir and channel are initially filled with a hardening substance, which can include a plastic material having a high thermal expansion coefficient.
The inkjet printhead also comprises at least one internal heater, which is predisposed within the reservoir and/or the channel adjacent the hardening substance. The internal heater(s), when operated in a first mode, are adapted to cause the hardening substance to soften and flow. Furthermore, the internal heater(s) are configured to cause the plastic material to expand and enter the nozzle cavity when activated in order to modify the nozzle cavity space; and, when cooled, to remain in the cavity. That is, the plastic material is adapted to either form a protrusion in the nozzle cavity, thereby decreasing the nozzle cavity space. Alternatively, when heat is applied to the heater(s) in a second mode of operation, the plastic material is caused to flow and increase the nozzle cavity space. Thus, the hardening substance is adapted to internally alter the nozzle cavity space and cause ink ejected from the nozzle opening to be deflected with regard to a desired amount of compensation.
In accordance with yet another embodiment, disclosed is a method of modifying the nozzle cavity space of an inkjet printhead having at least one reservoir with a channel so as to compensate for the effects of defects in the printhead by altering the direction of ink drops ejected from a nozzle. Initially, the printhead is tested to determine the ink stream directionality onto a receiver medium, such as paper, from a nozzle opening. Variability in the direction of the ink drops ejected from a nozzle of the inkjet printhead caused by manufacturing defects is then identified. Thus, the amount of misdirection from a nozzle of an inkjet printhead can be quantified and the amount of compensation desired in the direction of ink,ejected from the nozzle opening can be determined.
The method comprises the step of causing ink ejected from the nozzle opening to be deflected with regard to the desired amount of compensation. In one embodiment, an internal heater is activated within a reservoir. The application of heat via the internal heater causes a hardening substance, or plastic material, within the reservoir and channel to protrude into the nozzle cavity. A change in temperature is sustained until the desired amount of compensation is obtained. Thus, the nozzle cavity space is internally modified in order to correct the misdirection of ink ejected from the nozzle opening.
Once the desired amount of compensation has been achieved by modifying the nozzle cavity space, the hardening substance within the channel is cooled. The decrease in temperature causes the hardening substance to freeze in a protruding state, thus decreasing the nozzle cavity space. The step of cooling is then followed by the step of deactivating the internal heater. The elimination of heat causes the hardening substance within the reservoir to harden.
According to another embodiment, the step of causing ink ejected from the nozzle opening to be deflected with regard to the desired amount of compensation further includes the step of heating and cooling the hardening substance, preferably using two heaters, one within the reservoir and one within the channel. This, in turn, causes the hardening substance, or plastic material, within the channel to form a recession. Thus, the nozzle cavity space is increased in order to correct the misdirection of ink ejected from the nozzle opening.
Technical advantages of the present invention include a cost effective method of compensating for the effects of defects in inkjet printheads that would otherwise result in misdirection of ink drops ejected from the nozzles. As such, printing artifacts caused by irregularities in the ink drops landing onto a receiver medium are eliminated.
Other technical advantages include the increase in manufacturing yields as printheads that would be typically discarded can be repaired and used.